Effect of carbohydrate moieties on the folate hydrolysis activity of the prostate specific membrane antigen

BACKGROUND. Prostate specific membrane antigen or PSMA has been recognized as one of the important cellular markers for prostate cancer, the expression of which is enhanced many fold in prostate cancer and other tumor neovasculature. PSMA is a type II membrane glycoprotein with a short cytoplasmic N-terminal region, a transmembrane domain, and a 701 amino acid extracellular portion with 10 potential N-linked glycosylation sites. PSMA is a folate hydrolase, which cleaves terminal glutamates from poly- and gamma-glutamated folates; and NAALADase, which hydrolyses alpha-glutamate-linked dipeptide, N-acetyl-aspartyl-glutamate (NAAG) and is a glutamate carboxypeptidase. METHODS. In our study we have used various enzymes or site directed mutagenesis to remove sugar molecules from PSMA protein and studied its folate hydrolase function. We have performed a biochemical characterization of N-linked glycosylation of the various mutant proteins. RESULTS. PSMA protein expressed in different prostate cancer cell lines is differentially glycosylated. Removal of sugar residues either enzymatically or by mutagenesis abolishes the enzyme activity of PSMA protein completely. CONCLUSION. N-linked carbohydrate structures are important for the folate hydrolase function of the protein. Removal of sugars partially or completely causes PSMA to be enzymatically inactive, improperly folded, resulting in increased rate of degradation. (C) 2003 Wiley-Liss, Inc.